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doc: Add disable_cleaner requirements documentation

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Signed-off-by: Robert Baldyga <robert.baldyga@intel.com>
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Robert Baldyga 2022-04-22 00:26:34 +02:00
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---
group: disable_cleaner
---
Disabling cleaner is a feature that is intented to be used with read cache
(Write-Thourgh and Write-Around modes), where no dirty data is produced, so
the cleaning is not necessary. Its major benefit is metadata memory footprint
reduction, which is result of not allocating "cleaning" metatada section,
that constitutes about 20% of total metadata capacity.
While it is possible to use disable_cleaner option Write-Back and Write-Only
modes, it may result in performance degradation due to necessity to perform
cleaning on eviction.
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
title: Setting cleaner_disabled mode
id: set_cleaner_disabled
---
It shall be possible to select cleaner_disabled mode during cache attach
operation by setting apropirate field in attach config.
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
title: Loading cache cleaner_disabled mode
id: load_cleaner_disabled
---
The cleaner_disabled setting should be preserved in cache metadata, i.e. on the
cache load/recovery the setting should be restored to the value it had before
the cache stop.
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
title: Metadata "cleaning" section allocation
id: cleaning_section_alocation
---
When disable_cleaner option is selected the "cleaning" section in OCF metadata
shall not be allocated neither in DRAM nor on the cache drive.
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
title: Starting with NOP cleaning policy
id: starting_with_nop_policy
---
When attaching/loading cache with disable_cleaner option selected, the cleaning
policy shall always be NOP.
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
title: NOP cleaning policy enforcement
id: nop_enforcement
---
When disable_cleaner option is selected it shall not be possible to change the
cleaning policy to other than NOP.
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
title: Default setting
id: default_setting
---
By default the disable_cleaner option shall not be selected.